Wireless communication systems, such as GSM and the 3rd Generation (3G) of mobile telephone standards and technology, are well known. An example of 3G standards and technology is the Universal Mobile Telecommunications System (UMTS™), developed by the 3rd Generation Partnership Project (3GPP™) (www.3gpp.org).
The 3rd and 4th generations of wireless communications, and particular systems such as LTE, have generally been developed to support macro-cell mobile phone communications. Here the ‘phone’ may be a smart phone, or another mobile or portable communication unit that is linked wirelessly to a network through which calls are connected. Henceforth all these devices will be referred to as mobile communication units. ‘Calls’ may be data, video, or voice calls, or a combination of these. Such macro cells utilise high power base stations to communicate with wireless communication units within a relatively large geographical coverage area. The coverage area may be several square kilometers, or larger if it is not in a built-up area.
Typically, mobile communication units communicate with each other and other telephone systems through a network. In a 3G system, this is the ‘Core Network’ of the 3G wireless communication system, and the communication is via a Radio Network Subsystem. A wireless communication system typically comprises a plurality of Radio Network Subsystems. Each Radio Network Subsystem comprises one or more cells, to which mobile communication units may attach, and thereby connect to the network. A base station may serve a cell with multiple antennas, each of which serves one sector of the cell. Often a wireless communication system is described as comprising two parts: the network, and the mobile communication units.
FIG. 1 provides a perspective view of one prior art wireless communication system 100. The system of FIG. 1 comprises a network of base stations BS1-BS8. Only one mobile communication unit 105 is shown. In a real network, there may be anywhere from thousands to millions of mobile communication units.
A base station such as BS1 110 communicates with mobile communication unit 105. Base station BS1 100 allows mobile communication unit 105 to place calls through the network, and receive calls routed through the network to base station BS1 100.
Base station BS7 112 has been shown as having a coverage area 114. If base station BS7 had an omnidirectional antenna, and the terrain were flat, then coverage area 114 might be circular. However, the coverage areas of typical base stations depend on many variables, and may change with time.
Controller 190 manages calls within the wireless communication system 100. Controller 190 would be linked to all the base stations BS1-BS8 , but the links are not shown in order to keep FIG. 1 simple. Controller 190 may process and store call information from the base stations BS1-BS8 , plus many other base stations not shown in FIG. 1. In a UMTS network, controller 190 may be linked to the base stations BS1-BS8 via one or more Radio Network Subsystems.
Other known wireless communication systems include:
‘Mobile Location Estimation Based on Differences of Signal Attenuation for GSM Systems’, Lin and Juang, IEEE Trans, on Vehicular Technology, July 2005. This paper is available at:
http://www.cce.ntut.edu.tw/ezfiles/0/academic/43/academic 46991 5867233 59222.pdf
This publication uses measurements of the differences between signal strengths, received by a mobile communications unit. The signals considered are from omni-directional antenna. Parameters of the network are derived from a model called the “Cost-Hata” model.
‘Cellular Geolocation Employing Hybrid of Relative Signal Strength and Propagation Delay’, Liu and Lin, WCNC 2006 Proceedings.
This publication uses measurements of the differences between signal strengths, received by a mobile communications unit, and a measure of “Propagation Delay” for signals. The propagation model parameters are from the “Okumura-Hata” model. This publication uses a method of location estimation that assumes omni-directional antennae.
Patent application WO2010/083943A shows a further technique, which uses signal strength and timing data derived from the wireless communication unit itself, along with network configuration data provided by the network operator, to locate the wireless communication unit.
In conventional wireless communication systems, there is wide variation in the power levels of signals received from base stations. The variations depend on many issues, including location of the wireless communication unit and time. Conventional wireless communication systems also employ “Timing Advance”. This is a deliberate offset, introduced into communication signals. Timing advance is used to allow better synchronisation of received signals by various different mobile communication units, located at different distances from a base station. Hence timing advance generally varies as a function of distance from the transmitter. In a typical implementation, timing advance may be controlled for users who are further away from the transmitter. This feature should be available in LTE, but the reporting of it may not be activated. However, a measure of timing advance is available from measurements made by the mobile communication unit, in some cellular wireless communication systems.
Known cellular wireless communication systems have the disadvantages that:    a) Generally, the exact calculation of received power as a function of distance is not possible. This is a result of factors whose magnitude is unknown a priori, such as:
(i) The rate at which the power level attenuates with distance, and the attenuation of power at a reference distance.
(ii) The additional attenuation of signals due to a mobile communication unit being located in a vehicle, such as a car or train, or in a building.    b) Timing Advance is not a direct measure of distance. Its use is primarily for synchronisation of communication channels. As such its value depends not only on distance, but also on various effects such as multi-path fading, hardware timing delays and such like. Hardware timing delays may be in the base station, or in a repeater in the cable from the antenna. If so, they are usually inaccessible, and hence may only be known to the infrastructure operator. However, some equipment allows the delays to be programmable, and hence both controllable and known. Furthermore timing advance is a coarse measure, each unit of timing advance corresponding to a distance of ˜553 meters in GSM. Timing advance is however more precise in LTE, due to the higher symbol rate. It may be less than 100 meters in LTE.